Mud Pulsers
It is well known that the efficiency of oil well drilling operations can be significantly enhanced by monitoring various drilling parameters at the drill bit. For example, information about the location and orientation of the downhole drilling assembly is necessary for accurate control during directional drilling operations. Furthermore, downhole drilling parameters, such as torque and weight-on-bit, can be combined with new drill bit technologies to enhance penetration rates and to significantly reduce drilling costs.
In the late 1960's and early 1970's, various measurement-while-drilling (MWD) systems were developed to allow the measurement of downhole drilling parameters and the telemetry of these data to the surface. In the late 1970's, commercial MWD services were offered by several oil field service companies. Presently, all commercial MWD systems are similar in that the hydraulic fluid coupling between the drill bit and the mud pump at the surface is used as the data telemetry channel.
Conventional MWD technology involves the downhole measurement and encoding of drilling parameters and the modulation of a drilling fluid, normally referred to as the drilling mud, via a mud flow valve, normally referred to as a mud pulser, which is located near the drill bit. The mud pulser operates in either a negative mud pulse configuration or in a positive mud pulse configuration. In a negative mud pulse configuration, drilling mud is vented from the inside of the drill pipe to the annulus. In a positive mud pulses configuration, mud flow in the drill pipe is restricted. In either case, the positive and negative pressure fluctuations generated by the mud pulser propagate up the inside of the drill pipe, to the surface, where they are detected and decoded.
All commercial mud pulser devices are mechanically complicated, requiring a significant amount of energy to actuate the valve, involving complex pressure compensation schemes, and utilizing intricately shaped valve assemblies which are exposed to abrasive muds. As a result of this complexity, pulser assemblies exhibit a mean time between failure rates of approximately 200 hours and usually require total rebuilding after field service. Because of the significant energy requirements of the pulsers, the MWD systems are further complicated by complex high power electronic designs and turbine/generator assemblies. This complexity of conventional MWD systems results in reduced reliability and cost effectiveness of the MWD systems as a whole.